Population studies have demonstrated that the prevalence and incidence of coronary heart disease is inversely correlated with the plasma concentration of high density lipoprotein (HDL) cholesterol, suggesting that HDL may protect against atherosclerosis. This protection may be related to the ability of HDL to promote the "clearance" of cholesterol from cells of the artery wall. In support of this proposal are results from recent studies suggesting that cultured human skin fibroblasts and human arterial smooth muscle cells possess specific high-affinity HDL receptors on their cell-surfaces that are induced when cells become loaded with cholesterol. The objective of this research project is to assess the role of this HDL receptor in modulation of cellular cholesterol homeostasis in key cells of the artery wall: endothelial cells, arterial smooth muscle cells, and monocyte-derived macrophages. Different types of cultured cells will be used as experimental models, including human fibroblasts, human arterial smooth muscle cells, bovine endothelial cells, mouse peritoneal macrophages, and human monocyte-derived macrophages. Studies will be conducted to characterize HDL receptors on these cells, identify and characterize the specific ligand(s) that bind to the HDL receptor, and delineate the mechanisms by which HDL influences cell cholesterol homeostasis. Experiments will be designed to test for the ability of a suspected ligand to compete for binding of radiolabeled HDL to cells, to evaluate the effects of selective chemical modification of proteins and lipids on ligand-receptor interactions, to characterize the physical properties of the HDL receptor on intact cells and isolated membrane preparations, and to assess the effects of ligand-receptor interaction on the biochemical parameters related to cell cholesterol metabolism. Results from these studies should lead to a better understanding of the role of HDL in modulation of the cholesterol content of the artery wall, thus increasing our knowledge of the mechanisms involved in the pathogenesis of atherosclerosis.